Ken Livingstone claims Labour voters are telling him on the doorstep they will vote for Boris Johnson because “he makes them laugh”.

The admission comes as polls show almost one in 10 Labour supporters is considering voting for the incumbent.

A leaked email from campaign chief Tessa Jowell to her party’s MPs identified a “Labour gap” of voters who needed to be won back.

At a Westminster lunch, Mr Livingstone was pressed over why his natural supporters were turning to Mr Johnson.

He said: “Boris realised somewhere before he got into Eton that if he does this ‘oh I will shake my hair and have a joke’, he gets away with murder. I find people on the doorstep who are naturally Labour and they say to me ‘oh, he does make me laugh’.

“I didn’t come into politics to make you laugh. If I wanted to make you laugh I would have been a bloody stand-up comedian. I think we should have a compromise. Boris should let me be Mayor and I will support him to be the permanent host of Have I Got News For You.”

Mr Livingstone denied he was tired and his campaign lacked drive. He said: “I’ve lost a stone during the campaign. I went for my annual medical last week and my doctor almost had an orgasm I was so fit.”

Joking about his age, the 66-year-old former mayor said: “I’m doing what Cameron and Osborne want — I’m working longer.”

Ms Jowell asked MPs to call supporters and urge them not to vote for Mr Johnson. “We would like to send you a list of phone canvassing for our ‘Labour gap’ voters —people who told us they vote Labour at general elections, but are supporting Boris Johnson for the mayoralty,” she wrote.

“We need your experience and knowledge of London to persuade these voters to switch back to Labour for Ken.”